Transmission line system



Iatented Feb. 22, 1949 TRANSMISSION LINE SYSTEM Robert S. Wehner, PortJefferson, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application January 7, 1946, Serial No. 639,606

4 Claims.

The present invention relates to transmission line systems and, moreparticularly, to such systems for feeding a single antenna at two widelyspaced frequencies.

- An object of the present invention i the provision of a novel systemfor feeding and matching an antenna to a transmission line over twowidely spaced frequency bands.

- Another object of the present invention is the provision of a systemas aforesaid which permits the use of any desired type of matchingsection in the branches of the circuit.

A further object of the present invention is the provision of a singlesmall antenna for use on two widelt spaced frequency bands.

Still a further object of the present invention is the elimination ofthe necessity for using a plurality of antennas on aircraft.

The foregoing objects and others which may appear from the followingdetailed description are attained by providing a reentrant branchedtransmission line circuit from a transmitter capable of operating on apair of different frequencies to a single antenna. The branches of thetransmission line are so arranged that energy of one frequency passesentirely through one branch of the system while energy at the otherfrequency passes through the other branch. Thus, each branch may containits own individual impedance matching circuits for matching the antennato the feed line from the transmitter.

The present invention will be more fully understood by reference to thefollowing description which is accompanied by a drawing in whichreference number It denotes an antenna which is to be fed by means oftransmission line TL with one of two frequencies A1 and m from atransmitter (not shown) Antenna Iii, while illustrated as atwo-frequency stub antenna of the type described in more detail in mycopending application #636,932, filed December 22, 1945, need notnecessarily be of this type. A simple stub antenna may be used ifdesired.

The following description will be generally predicated on the assumptionthat a simple stub antenna is used. Any one interested in a furtherelucidation of the principles of operation of the two-frequency stubantenna is referred to the more complete description appearing in thecopending application referred to above.

The transmission line TL contains a pair of reentrant branches TL1 andTL2 connected in parallel relationship between points a and d. The innerconductor 29 splits into conductors 2| and 22 at point a and then againcomes together in a single conductor at point d. Branch TL1 contains ashort-circuited series line section l5, having a pair of parallel innerconductor l6 and I1, each a quarter wave long at the frequency A2 whichis the frequency adapted to be passed 2 through the other branch of thenetwork. Conductors It and I! are connected together and to the outershell I8 of the line section [5 at one end and connected in series withthe inner conductor 2| of transmission line TL at their other ends.Conductor 21 then passes through an impedance matching section MMdesigned to match the impedance looking out of the matching section IEto the impedance at the input of a second series line section 25 at thefrequency M. The second short-circuited series line section 25.,likewise having short-circuited quarter wave inner conductors i6 and I!and an outer casing I8 identical in dimensions and arrangement to theshort-circuited series line section l5, follows the matching section MM.From the second series section the transmission line TLi again entersthe main transmission line TL at point d and proceeds to the antenna 10.Branch TL: has the same reference numerals as applied to branch TLi butprimed in order to indicate that the construction is generally similarbut the actual dimensions are different. That is, the short-circuitedseries line sections l5 and 25 are designed to present high impedance atfrequencies which are adapted to be passed by the line TLl. The

matching section MM is designed to match the impedance looking out ofshort-circuited line section l5 into the short-circuited series of linesection 25.

In considering a signal of Wavelength A1 approaching the junction at (a)from the direction of the transmitter, the signal will take theright-hand branch at (a) into transmission line section TL1 for thefollowing reasons: I

Looking into TLZ from (a) is seen a very high impedance due to linesection l5, which, being shorted at its far end and one quarterwavelength long at A1, presents a very high impedance in the directionfrom (a) to Hi. I

Looking into the right-hand branch TLz is seen a matched impedance ofthe system since the matching section MM is designed to match theimpedance of the entire system beyond the matching section to the systemconsisting of the line section IS in series with the transmission lineTL leading from the transmitter. The transmitter is of course assumed tobe matched to the main transmission line TL.

Therefore, all or almost all of the current entering the junction (a) atthe wavelength M takes the right-hand branch passing in turn through theline section l5, through the matching section MM, through the linesection 25, through the junction (d) at the base of the antenna and intothe antenna l0. At the junction (41) almost all of the current passesinto the antenna since there is a very high impedance look- 3 ing in thedirection from (d) to the line section 25', this line section beingidentical with line section l" and, presenting a very high-impedance toenergyat a"wavelength A1 thereby preventing circulating current in thebranch circuit.

In similar manner it is evident that at thewavelength )\2 all or almostall of the: current will a go through transmission line TLz to theantenna I0, the line sections [5 and 25 'beingshorted quarter wave linesat this wavelength andcon actanceatfrequencies above resonance. At thewavelengthm the antenna is fed through branch:

The line section TLzwhich at thiswavelength is less than a quarterwavelength long introducesan inductive reactance in series with theantenna-at point (d). This inductance tends to cancel 'thecapacitivereactance of the antenna; Thus,-it is" easier to match the systembetween matching section MM and antenna ID to the transmissionline TLthrough-the matching section MM; Also matching section MM iseffective-overa broaderband of frequencies than would-otherwisebe-possible; Conversely for short wavelength- A2 at 1 which through thelefthand branch TLz, the line section 25 which at this wavelength ismore than a quarter wavelength long introduces'a capaci-- tive reactancein series with-theantenna. This capacitance tends-to cancel theinductive reactance of-the antenna making the system easier to matchbymeansof matching circuit Mk2, makingthe matchingcircuiteffect-ive over abroader While I haveillustrateda particular embodi ment of the presentinvention, it should be clearly understood-that it is not limitedthereto since many, modifications may be made in the severalelementsemployedand in their arrangement and it is thereforecontemplated to cover any such modifications asfallwithi'n-the spiritand scope of theinvention.

What is claimed is:

1. 'Anantenna feed line system for. operation at ,two frequenciesincludinga branched'reentranttransmission line from, a source of energyat; said; frequencies to a single antenna each branch of saidtransmission line. including a first and a,secondshortecircuited seriesline: section, the line, sections inieachbranch presenting. a high"impedance to, a. different.- one, -.of1saidi. fre-v quencies, a seriesmatching. sectioninterposedv between said series linesections in each.branch; said matching sections serving. to match the impedance of thefirst of said line sections to the the antenna is fed.

4 impedance of the second of said line sections and said antenna at theone of said two frequencies to which the linesections in said; branch donot present" a high'impedance:

2. An antenna feed line system for operation at two frequenciesincluding a branched reentr-ant transmission line from a source ofenergy at said frequencies to a single antenna, each branch ofsaid-transmission line including an innerconductorand outer shell andeach branch of said transmission line including a first and asecondshort-ci-rcuited series line section, the line sections in eachbranch presenting a high impedance to a different one of saidfrequencies, a series matching section interposed in each branch betweensaid seriesxline sections, said matching sections each serving to :matchthe ima; pedance of the firstof said line sectionsatoithe. impedance ofthe second of said linezsectionsz andLsaid antenna at theother-ofsaidtwdfrequencies; said series line sections each includinglapair of parallel conductors within an outer. Shel-1;

said conductors each having.allengthequalvto an odd multiple includingunity of a quarter wavelength-at'the frequency to whichiitisito present.a high impedance, said conductors being connected together and to saidshellat one pair of ad jacent ends, the other endsbeingconnected-inseries with-the inner conductor of its branch.-

3 An antenna feed line system for operationat twofrequencies including abranched reentranttransmission line from a source of energy at saidfrequencies to a single-antenna, each branch of said transmission lineincluding means presenting a high impedance to a different one of saidfrequencies, each'branch also including a series matching section, eachof said matching sect-ions serving-to match the impedances in each 1branch 'for the frequency which it is to pass.

4.--In an antenna feed line system for'operation at two'frequenciesincluding "a branched re-- entrant transmissionline from a source ofenergy at said frequencies to asingle' antenna, said transmissionlineincluding an outer shell andinner conductor and each-branch of saidtransmission-line including impedance elements presenting a'highjimpedance to one of said frequencies andhaving a minimum effect on" theother "of said frequencies, said impedance elements', including apair'of parallel conductors within an outer'shell, said conductors'eachhav-'" ing a length equal to an odd multiple including unity of' aquarter wavelengthat the frequency to' which itis to present a highimpedance," said conductorsbeing connected together. and to saidshell-at one pair of adjacent'ends, the other ends being connectedin'series' with the inner conductor of its branch.

ROBERTS. W'EHNERT REFERENCES CITED The following references are ofrecord-inthe le. of this patent:

UNITED STATES PATENTS Number Name Date 2,128,400 Carter Aug. 30,19382,190,131.: Alford Feb. 13, 1940 2,212,214, Smith Aug. 20, 1940.2,318,237 Lin-denblad May 4, 1943 2,341,408 Lindenblad Feb.-8,- 1944=

